Elst–Pizarro was reported in 1979 as minor planet 1979 OW7, with its image on a photographic plate being completely stellar in appearance. Its orbit remains entirely within the orbits of Mars and Jupiter, with eccentricity 0.165, typical of a minor planet in the asteroid belt. However, the images taken by Eric W. Elst and Guido Pizarro in 1996, when it was near perihelion, clearly show a cometary tail. Since this is not normal behaviour for asteroids, it is suspected that Elst–Pizarro has a different, probably icy, composition. The cometary nature of Elst–Pizarro was first discovered when a linear dust feature was observed with the ESO 1-metre Schmidt telescope at La Silla Observatory on 7 August 1996.[8][11]

Subsequently, around the next perihelion in November 2001, the cometary activity appeared again, and persisted for 5 months.[8]

Castalia is a proposed mission concept for a robotic spacecraft to explore Pizarro and make the first in situ measurements of water in the asteroid belt, and thus, help solve the mystery of the origin of Earth's water.[13] The lead is Colin Snodgrass, from The Open University in the UK. Castalia was proposed in 2015 and 2016 to the European Space Agency within the Cosmic Vision programme missions M4 and M5, but it was not selected. The team continues to mature the mission concept and science objectives.[13] Because of the construction time required and orbital dynamics, a launch date of October 2028 was proposed.[13]